fix(eslint-plugin): [promise-function-async] use a different error message for functions with promise and non-promise types (#10950)

* initial implementation

* tests

* snapshots

* cover overloading case

* Update packages/eslint-plugin/src/rules/promise-function-async.ts

Co-authored-by: Josh Goldberg ✨ <[email protected]>

* snapshots

* Update packages/eslint-plugin/src/rules/promise-function-async.ts

Co-authored-by: Kirk Waiblinger <[email protected]>

* snapshots

---------

Co-authored-by: Josh Goldberg ✨ <[email protected]>
Co-authored-by: Kirk Waiblinger <[email protected]>

Author: 3 people

Diff for: packages/eslint-plugin/src/rules/promise-function-async.ts

@@ -23,7 +23,7 @@ export type Options = [
     checkMethodDeclarations?: boolean;
   },
 ];
-export type MessageIds = 'missingAsync';
+export type MessageIds = 'missingAsync' | 'missingAsyncHybridReturn';
 
 export default createRule<Options, MessageIds>({
   name: 'promise-function-async',
@@ -37,6 +37,8 @@ export default createRule<Options, MessageIds>({
     fixable: 'code',
     messages: {
       missingAsync: 'Functions that return promises must be async.',
+      missingAsyncHybridReturn:
+        'Functions that return promises must be async. Consider adding an explicit return type annotation if the function is intended to return a union of promise and non-promise types.',
     },
     schema: [
       {
@@ -164,10 +166,20 @@ export default createRule<Options, MessageIds>({
           ),
         )
       ) {
+        const isHybridReturnType = returnTypes.some(
+          type =>
+            type.isUnion() &&
+            !type.types.every(part =>
+              containsAllTypesByName(part, true, allAllowedPromiseNames),
+            ),
+        );
+
         context.report({
           loc: getFunctionHeadLoc(node, context.sourceCode),
           node,
-          messageId: 'missingAsync',
+          messageId: isHybridReturnType
+            ? 'missingAsyncHybridReturn'
+            : 'missingAsync',
           fix: fixer => {
             if (
               node.parent.type === AST_NODE_TYPES.MethodDefinition ||

Diff for: packages/eslint-plugin/tests/docs-eslint-output-snapshots/promise-function-async.shot

@@ -825,7 +825,7 @@ function promiseInUnionWithoutExplicitReturnType(p: boolean) {
       `,
       errors: [
         {
-          messageId,
+          messageId: 'missingAsyncHybridReturn',
         },
       ],
       output: `
@@ -836,6 +836,65 @@ async function promiseInUnionWithoutExplicitReturnType(p: boolean) {
     },
     {
       code: `
+function test1(): 'one' | Promise<'one'>;
+function test1(a: number): Promise<number>;
+function test1(a?: number) {
+  if (a) {
+    return Promise.resolve(a);
+  }
+
+  return Math.random() > 0.5 ? 'one' : Promise.resolve('one');
+}
+      `,
+      errors: [
+        {
+          messageId: 'missingAsyncHybridReturn',
+        },
+      ],
+      output: `
+function test1(): 'one' | Promise<'one'>;
+function test1(a: number): Promise<number>;
+async function test1(a?: number) {
+  if (a) {
+    return Promise.resolve(a);
+  }
+
+  return Math.random() > 0.5 ? 'one' : Promise.resolve('one');
+}
+      `,
+    },
+    {
+      code: `
+class PromiseType {
+  s?: string;
+}
+
+function promiseInUnionWithoutExplicitReturnType(p: boolean) {
+  return p ? new PromiseType() : 5;
+}
+      `,
+      errors: [
+        {
+          messageId: 'missingAsyncHybridReturn',
+        },
+      ],
+      options: [
+        {
+          allowedPromiseNames: ['PromiseType'],
+        },
+      ],
+      output: `
+class PromiseType {
+  s?: string;
+}
+
+async function promiseInUnionWithoutExplicitReturnType(p: boolean) {
+  return p ? new PromiseType() : 5;
+}
+      `,
+    },
+    {
+      code: `
 function overloadingThatCanReturnPromise(): Promise<number>;
 function overloadingThatCanReturnPromise(a: boolean): Promise<string>;
 function overloadingThatCanReturnPromise(